1. Field of the Invention
The present invention relates to expandable phenolic resin-coated composite beads which are moldable by heating in a mold, to a process for production thereof and to a process for molding composite molded products by foaming phenolic resins using the composite beads.
2. Description of the Prior Art
An expandable phenolic resin initial condensation product obtained by mixing a novolac type or solid resol type phenolic resin initial condensation product with a desired amount of a foaming agent and, optionally, a hardening agent is usually powdered. The particle size of the powder and the bulk specific gravity are generally greater than 100 mesh and smaller than 1, respectively. In order to obtain a composite molded product by mixing the composition with other aggregates and foaming the mixture by heating, it is required that the size of the aggregates be smaller than 1 mm and their bulk specific gravity be almost the same as that of the aforesaid composition. Otherwise, it is difficult to obtain a homogeneous mixture.
Even though such a mixture is foamable by heating, it is extremely difficult to obtain phenolic resin molding products having a uniform degree of expansion. The reason is because the use of larger aggregates results in confinement of the powdery composition at the lower part of a mold. Bulk specific gravities which are different from each other result in phase separation.
Further, as is disclosed in Japanese Published Examined Patent Application No. 29774/73, it is known that composite molding products of foamed phenolic resins are prepared by filling a mold with inorganic aggregates coated with a liquid, expandable resol type phenolic resin composition and then heat-foaming. However, the aggregates coated with the liquid composition stick to each other so that the fluidizability becomes poor, and an efficient operation of filling the mold becomes difficult. At the same time, filling the mold with a given amount of the composition becomes difficult. In addition, means for transporting the particles with air cannot be used since the respective aggregates stick to each other and do not form independent particles. Furthermore, there are disadvantages that the liquid composition cannot be uniformly coated on each of the aggregates and hardening of the coated composition is initiated at normal temperature. A hardening reaction proceeds at normal temperature with liquid, resol type expandable phenolic resin initial condensation products and even if the products are stored at lower temperatures, the storage is effective only for about 2 months. Additionally, composite molded products prepared by heating the coated aggregates in a mold produces many voids scattered in the phenolic resin-expanded layer, resulting in a non-uniform density.